Implantable cardiac stimulation devices have been developed to treat a wide variety of patient cardiac arrhythmias. Implantable cardiac stimulation devices typically include an implantable battery-powered stimulation pulse generator and microprocessor-based controller which are encased in a biocompatible housing. One or more sensing/stimulation electrodes are typically positioned in contact with corresponding cardiac tissue and are interconnected via one or more implantable leads with the controller and stimulation pulse generator. The device monitors the physiological condition of the patient and selectably provides therapeutic stimulations when indicated to attempt to restore desired cardiac function.
Simpler devices may monitor and/or treat only a single chamber of the patient's heart, whereas more sophisticated devices are capable of sensing and delivering therapy to multiple chambers and may be referred to as dual or multi-chamber devices. As one example of the capabilities of such devices, a multi-chamber device can be capable of sensing the activity in both a patient's atrium and ventricle, as well as delivering independent therapy to one or more of the chambers when indicated. The timing of stimulation delivery is generally controlled by the device's controller as a function of both sensed intrinsic events as well as evoked or stimulated cardiac activity. The timing of therapy delivery is typically controlled to accommodate refractory periods following depolarization events as well as to accommodate and synchronize the activity of the multiple chambers of the heart to improve the pumping performance or hemodynamic characteristics of the heart.
As one particular example, many multi-chamber devices are capable of monitoring ventricular activity and providing therapeutic stimulation to the ventricle based on activity in the corresponding atrium. For example, such devices may be capable of monitoring the occurrence of an intrinsic depolarization in an atrium or alternatively tracking the delivery of a therapeutic stimulation to the atrium and then subsequently monitoring the activity of the corresponding ventricle and delivering a subsequent ventricular stimulation if an intrinsic ventricular depolarization does not occur within an indicated time window. This time window is generally referred to as an atrio-ventricular (AV) delay and marks a duration following either of an intrinsic atrial depolarization or an evoked or stimulated atrial depolarization at the end of which a ventricular stimulation will be provided if an appropriate intrinsic or AV conducted depolarization does not occur. In certain cases, the AV delay is programmed to a fixed value and in other cases is variable based on the heart rate which the device is currently seeking to maintain.
A desirable feature of implantable cardiac stimulation devices is that they be capable of dynamically accommodating variations in the patient's metabolic need. For example, implantable devices may be capable of tracking or monitoring a patient's activity level and adjusting one or more operating parameters of the device, such as a heart rate which the device maintains, to correspond with the changes in the patient's metabolic need. Such devices are generally referred to as rate responsive devices. Thus, in certain cases, rate responsive devices can vary other operating parameters of the device, such as an AV delay, with changes in a heart rate which the device maintains. In a normal healthy person, a variety of physiological feedback mechanisms act to adjust the function of a variety of physiological processes, including cardiac activity, to track ongoing changes in the person's status, such as activity level, periods of sleep or rest, physical and/or emotional stress, etc. A difficulty arises, however, with patients having impaired health, for example patients for whom an implantable cardiac stimulation device is indicated. More particularly, certain feedback or automatic control mechanisms which are generally intact in the normal healthy person are either not well understood or have proven difficult to replicate with an implantable device, particularly when the device is assuming a significant proportion or complete control of the patient's cardiac activity.